1. Field of the Invention
This invention relates generally to the field of free-piston Stirling machines and, more particularly, to a head structure and cooperating clamping assembly for allowing access to the thermal regenerator packing in the head of a Stirling machine between tests of the machine and also facilitating final, close-out welding for hermetically sealing the machine after testing is concluded.
2. Description of the Related Art
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because of advantages they offer. In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Stirling machines cyclically shuttle a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, regenerator and heat rejecter. The shuttling is commonly done by pistons reciprocating in cylinders and cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because the spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm sink.
Stirling machines can therefore be designed to use the above principles to provide either (1) an engine having pistons driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space, or (2) a heat pump having pistons cyclically driven by a prime mover for pumping heat from the expansion space to the compression space. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the pistons are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. Free-piston Stirling machines are constructed as mechanical oscillators and one of its pistons, conventionally identified as a displacer, is driven by the working gas pressure variations in the machine. The other piston, conventionally identified as the power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine. Free piston Stirling machines offer numerous advantages including the control of their frequency and phase and their lack of a requirement for a seal between moving parts to prevent the mixing of working gas and lubricating oil.
As known to persons skilled in the Stirling art, the regenerator of a Stirling machine consists of a porous, heat energy storage medium, typically a metal, with interconnected interstices that form a tortuous path through which the working gas flows in alternately opposite directions between the expansion space and the compression space. As the working gas flows through the regenerator, thermal energy is transferred between the regenerator medium and the working gas. A regenerator greatly improves the efficiency of a Stirling machine. Modern regenerator materials include fine porous metal wires or particles and non-woven, metal wire agglomerations.
The design and construction process for fabricating a Stirling machine so it operates according to particular operating criteria commonly requires multiple, sequential replications of the steps of extensive testing followed by modification of the design followed by testing of the modified design. The characteristics of the regenerator material that is packed in the regenerator cavity formed in the head of the Stirling machine have a substantial effect upon the operating characteristics of the Stirling machine. The regenerator material characteristics include the chemical composition of the material, the physical shape or morphology of the material, the density of the material and the size and shape of the passages through the material. Therefore, during the repeated design and testing phase, it is desirable to have convenient access to the internal regenerator packing material prior to final hermetic close-out of the machine. The convenience of acquiring access to the regenerator cavity so that the regenerator packing may be replaced or adjusted and the convenience of reassembly and hermetic sealing for further testing contribute to reducing the time, labor and cost of the design and testing phase.
After the design and testing phase is completed, the machine is then permanently hermetically sealed by performing a final close-out weld on the head of the machine which is desirably accomplished without disturbing the regenerator material.
It is therefore an object and feature of the invention to provide a head structure and associated clamping structure to provide a clamping apparatus that allows convenient temporary access to the regenerator packing of a Stirling machine, that allows the head to be conveniently resealed for further testing and that allows subsequent convenient access for further modification of the regenerator packing.
A further object and feature of the invention is to not only allow the previously described repetitions of regenerator packing modification followed by testing but additionally provides a structure on the head of the Stirling machine that facilitates final close-out welding of the machine's head without disturbing or damaging the regenerator cavity or the regenerator packing material in the cavity.